To establish a call connection in a communication network, a call setup procedure must be performed to determine the transmission route for the call connection and to reserve network resources for the call connection.
A call setup can be delayed for various reasons. A signaling route may be congested causing a signaling node to perform a scanning or a polling attempt in the communication network; network resources may be occupied by ongoing call connections; or, in a mobile communications network, establishment of a radio link may be delayed.
To avoid assigning resources to a call that is never completed, the duration of the call setup is supervised. To that end call setup supervision timers are provided in communication networks. When a call originating node initiates a call setup procedure, and a call setup message is transmitted towards a chain of nodes that transmit the call setup message, a supervision timer is started in the call originating node and in each of the other nodes that transmit the call setup message. When a response message is not received within a predefined expiration period, the call is released. When a response message is received within the expiration time, the supervision timer is reset.
More than one supervision timer can be provided in a node. For example, in the Bearer Independent Call Control (BICC) protocol, network timers T7 and T9 may be provided. The timer T7 is started in the call originating node and in each of the chain of nodes transmitting the call setup message during the call setup when an Initial Address Message (IAM) is sent towards a call terminating node. The timer T7 is reset when an Address Complete Message (ACM) is received, indicating that a network address of the terminating node has successfully been determined. When no ACM is received during the predefined expiration time of the T7 timer, the call is released. A second timer T9 is started when the ACM message is received in the call originating node, and is reset when an Answer (ANM) message is received. When no ANM message is received within the predetermined expiration time of the T9 timer, the call is released.
To prevent releasing a call when a specific reason for the prolongation of a call setup has been determined, excessive delay indication signals have been introduced. Reasons for a prolongation of a call setup may be that the network is congested. An excessive delay timer may be provided that initiates sending of an excessive delay indication signal when the timer expires.
A first excessive delay indication signal may be embodied as an early Address Complete Message (eACM) that is formatted as an ACM message. Like an ACM message, an eACM message stops the T7 timer and starts the T9 timer. Further excessive delay indication signals may be embodied as Call Progress (CPG) messages that restart the T9 timer.
Reference is now made to FIG. 1, which illustrates a mobile network 1000 and IAM messages sent during call setup in the mobile communication network. The mobile communication network 1000 comprises an originating node (ON) 111 within a originating network domain (OND) 100 and a destination network domain (DND) 140, that are interconnected by a plurality of Mobile Switching Centers (MSCs) 131, 132, and 133 forming a signaling path. Each of the MSCs includes an incoming protocol handler and an outgoing protocol handler. In particular, MSC 131 includes an incoming protocol handler 101 and an outgoing protocol handler 102; MSC 132 includes an incoming protocol handler 103 and an outgoing protocol handler 104; and MSC 133 includes an incoming protocol handler 105 and an outgoing protocol handler 106.
When a call is set up, the originating node 111 sends an IAM message 121 towards the destination network domain 140 and starts a supervising timer for supervising the duration of the call setup duration. The call setup is further continued by forwarding IAM messages 122, 123, and 124 towards the destination network domain 140.
Reference is now made to FIG. 2, which is a signaling sequence diagram depicting message within the mobile network of FIG. 1. Equally labeled elements refer to the respective elements in FIG. 1. In particular, FIG. 2 depicts the transmission and processing of IAM messages within the signaling path of FIG. 1 as well as the generation and conventional handling of excessive delay indication signals.
Upon reception of the IAM 121 in the incoming protocol handler 101 of MSC 131, an excessive delay timer for initiating the generation of an excessive delay indication signal is started in step 211, and IAM message 201 is forwarded to the outgoing protocol handler 102 within MSC 131. The outgoing protocol handler, in turn, starts a further timer for generating an excessive delay indication signal in step 212, and forwards IAM message 122 to MSC 132, which receives the IAM message 122 on its incoming protocol handler 103.
In MSC 132, a corresponding procedure is performed (i.e., the incoming protocol handler 103 starts its timer for initiating the generation of excessive delay indication signals in step 213 and forwards IAM 202 to its corresponding outgoing protocol handler). The outgoing protocol handler responds by forwarding IAM message 123 towards MSC 133 and starts its associated excessive delay timer in step 214.
Correspondingly, MSC 133, receives IAM 123 on its incoming protocol handler 105, starts the associated excessive delay timer in step 215, and forwards IAM 203 internally to outgoing protocol handler 106. The outgoing protocol handler, in turn, forwards IAM 124 and starts its excessive delay timer at step 216.
Accordingly in each of the incoming and outgoing protocol handlers of MSCs 131, 132, and 133, excessive delay timers are started.
When call setup is not completed within the predetermined expiration time of one of the excessive delay timers, an excessive delay indication signal is generated within the corresponding protocol handlers. The excessive delay indication signals restart a timer in the originating node 101 and in the other transmitting nodes 132 and 133 that supervise the duration of the call setup
As depicted in FIG. 2, when the timer for incoming protocol handler 101 within MSC 131 expires at step 221, an eACM message 231 indicating excessive delay is generated and transmitted towards the call-originating end of the signaling path of FIG. 1. Accordingly eACM messages 232-236, each indicating an excessive delay are generated in the protocol handlers 102-106, respectively, and are transmitted towards the call originating end of the signaling path.
Thus depending on the call setup conditions, a large number of eACM messages indicating an excessive delay can be generated and transmitted in the mobile network 1000. Sending a plurality of messages with excessive delay indication not only unnecessarily consumes signaling capacity, it can also lead to call release because the limit for the number of restarts of the timer T9 is exceeded.
Accordingly there is a need in the state of the art to limit the signaling load during a delayed call setup in a mobile communication network.